Not applicable.
The invention relates to a detector for a scanning electron microscope, in particular for a scanning electron microscope with variable pressure, and a scanning electron microscope with such a detector. By xe2x80x9cscanning electron microscope with variable pressurexe2x80x9d, or HPSEM, is to be understood a scanning electron microscope with which operation is possible with gas in the sample chamber at a pressure of at least 0.1 Pa. In HPSEMs, there is usually used as detector a collector electrode with following operational amplifier, or a gas scintillation detector. The latter consists of a light guide with a following photomultiplier. In both cases, a secondary electron cascade in the gas is required. Arrangements which use a secondary electron cascade are described in, for example, U.S. Pat. Nos. 4,785,182, 5,396,067, 5,677,531, WO 99/27559, JP 2236939, JP 2276846, and JP 2273445, and also in the article by G. Danilatos in the journal Advances in Electronics and Electron Physics, Vol. 78, pp. 1-102, 1990. The following problems arise in connection with the secondary electron cascade:
1. The amplification factor and the secondary electron cascade are limited by flash-overs.
2. In the present HPSEMs with collector electrode, the final pressure limiting aperture is at a similar potential to the collector electrode, i.e., the electrode at the end of the secondary electron cascade. The secondary electron cascade therefore has to take place for the most part on the same path section in the gas along which the primary electron scattering takes place (in the reverse direction). The pressure and the gas section therefore cannot be freely chosen, but their product must be large enough in order to obtain a sufficient amplification factor of the secondary electron cascade, even when the pressure or the gas section otherwise often do not at all actually have to be so large. Correspondingly, under these conditions, an undesirably strong primary electron scattering has to be accepted. This disadvantage also occurs with detection in the beam guiding tube.
3. In HPSEMs with a collector electrode and a following operational amplifier, no high scanning speeds are possible. At low scanning speeds, not even normal scanning speeds are possible, such as are required for alignment. The reason for this is that the time constants of the operational amplifier are too large for these scanning speeds at too high an amplification factor of the operational amplifier.
4. In both HPSEMs with gas scintillation detectors and also HPSEMs with collector electrode, the efficiency of the detection system is not fully adequate. A worsened signal/noise ratio and a greater damage to the specimen by the beam are the consequences, due to which the carrying out of many tasks is frustrated.
The present invention has as its object to provide an improved detector for HPSEMs with which at least a part of the above-mentioned problems is eliminated. The object of the invention is furthermore to provide a HPSEM with such an improved detector. These objects are attained by a scanning electron microscope that operates with gas in a sample chamber having a beam guiding tube for primary electrons, a sample chamber, a sample holder arranged in the sample chamber, a final pressure limiting aperture through which the primary electrons enter the sample chamber, a first electrode at a positive potential with respect to the sample holder and the final pressure limiting aperture for acceleration of secondary electrons emergent from a sample received by the sample holder, the first electrode being arranged outside the beam guiding tube, and at least one second electrode comprising an end facing toward the sample holder that is at a smaller distance from the sample holder than the first electrode and is at a potential that is between the potential of the first electrode and the potential of the sample, or is at the potential of the sample. The second electrode surrounds the first electrode and is substantially in the form of a funnel having a funnel tip toward the sample.
These objects are also attained by a scanning electron microscope that operates with gas in a sample chamber, having a sample chamber, a sample holder arranged in the sample chamber, the sample holder having a sample potential, a final pressure limiting aperture through which the primary electrons enter the sample chamber, and an electrode arranged outside the beam guiding tube. The electrode is electrically poorly conducting and comprises at least two contacts, a first one of the at least two contacts having a first potential and a second one of the at least two contacts having a second potential. An end of the electrode facing toward the sample holder is at an electrical potential that is between a higher one of the first and second potentials and the sample potential, or at the sample potential. The contact with the higher one of the first and second potentials is at a positive potential with respect to the sample holder and the final pressure limiting aperture.
These objects are also attained by a detector for secondary electrons in a scanning electron microscope with high pressure in a sample chamber with the use of a secondary electron cascade. At least one electrode with low electrical conductivity is provided, which extends along an elongate interspace or elongate cavity. In an inlet-side region within or in front of the cavity or interspace, the at least one electrode can have a potential applied such that a high amplification for secondary electrons results, and an elongate volume region with a reduced amplification factor for secondary electrons adjoins this inlet-side region.
These objects are also attained by a detector for secondary electrons in a scanning electron microscope with high pressure in a sample chamber with the use of a secondary electron cascade. A plurality of electrodes are provided that extend along an elongate interspace or elongate cavity. In an inlet-side region within or in front of the cavity or interspace, the electrodes can have a potential applied such that a high amplification for secondary electrons results, and an elongate volume region with a reduced amplification factor for secondary electrons adjoins this inlet-side region. The application of potential to the electrodes in the elongate volume region takes place such that an adjacent electrical field counteracts a tenuation of the secondary electron cascade due to impacts in the gas and due to drifting of the secondary electrons to the walls, so that a high but uncritical ionization density remains sustained.
A scanning electron microscope according to the invention has, like the known HPSEMs, a beam guiding tube for the primary electrons, with a final pressure limiting aperture on the sample side through which the primary electrons enter the sample chamber; a sample chamber; a sample holder in the sample chamber; and a first electrode which is at a positive potential relative to the sample holder and the final pressure limiting aperture of the beam guiding tube. The potential difference between the sample and the first electrode serves to accelerate secondary electrons which are released by the primary electrons from the sample received in the sample holder, the known secondary electron cascade being formed by the impact of these accelerated secondary electrons with the surrounding gas molecules and leading to an amplification of the secondary electron current.
In the meaning of the present application, the region between the electron source and the final pressure limiting aperture is termed the beam guiding tube.
In a first embodiment of the invention, at least one second electrode is provided, the end of which facing the sample holder is spaced closer apart from the sample holder than is the first electrode. This second electrode is at a potential that is between the potential of the first electrode and the potential of the sample, or at the potential of the sample.
In a second embodiment of the invention, a single electrode with low electrical conductivity is provided, along which a potential difference is established due to two different applied potentials, in the manner of a resistance chain. An electrode with varying potential along its surface results due to this potential difference.
The potential difference between the sample holder and the first electrode is affected by means of the second electrode, or by the changing potential along the electrode, so that an increased volume arises with high but uncritical ionization density.
A spatially varying ionization density is produced in the gas by the secondary electron cascade, and is dependent on the geometry and applied potentials of the electrodes that are present. When too high an ionization density is locally produced in the gas, flashovers occur. Besides electrons, the photons of gas scintillation are also principally produced in the region of high ionization density.
The functional principle on which the invention is based consists of producing an enlarged volume with high but uncritical ionization density in order to obtain a higher amplification factor for the secondary electrons, or a stronger photon signal. An ionization density is here termed xe2x80x9cuncriticalxe2x80x9d when no flashovers occur yet.
The enlarged region with high but uncritical ionization density is according to the invention bounded by electrodes so that this region is delimited relative to the rest of the sample chamber. This results from the fact that these electrodes delimiting the region simultaneously serve for the production of the enlarged region with high but uncritical ionization density.
A portion of the region with high ionization density can then also be situated outside the delimited region, for example in the form of a secondary electron cascade occurring in the delimited region. Here however in all, at least half of the volume with higher ionization density is nevertheless to be situated in the region which is delimited by the electrodes from the rest of the sample chamber.
The region delimited by the electrodes is situated outside the region enclosed by the beam guiding tube and delimited from the sample chamber by the final pressure limiting aperture. This has two advantages over an arrangement in which the enlarged region with higher but uncritical ionization density above the final pressure limiting aperture, and thus in the beam guiding tube. In the first place, no measures have to be undertaken so that the secondary electrons get through the pressure limiting aperture, which is problematic especially at large working distances and with small aperture diameters of the final pressure limiting aperture. Secondly, however, high primary electron scattering above the final pressure limiting aperture, and thus in the beam guiding tube, must above all not be accepted, which is inevitably the case when a high ionization density is to be attained in this region with a secondary electron cascade. A gas section with high primary electron scattering above the final pressure limiting aperture cannot be shortened by means of a small working distance, in contrast to a gas section below the final pressure limiting aperture. In the invention, by the arrangement outside the beam guiding tube of the region delimited by the electrode, it is possible to work with more moderate and lower primary energy, with the known advantages associated therewith.
The invention""s concept of an enlarged region with high but uncritical ionization density that is situated as a region delimited above the sample chamber by electrodes and outside the beam guiding tube, can be implemented in various ways:
In one embodiment, the volume with high but uncritical ionization density is enlarged perpendicularly to the direction of propagation of the secondary electron cascade. In an alternative embodiment, the volume with high but uncritical ionization density is enlarged in the direction of propagation of the secondary electron cascade. Mixed forms are furthermore also possible, in which the volume with high but uncritical ionization density is enlarged both perpendicularly of, and also in, the direction of propagation of the secondary electron cascade.
In the known HPSEMs with collector electrode, the ionization density reaches its maximum value close beneath the aperture of the final pressure limiting aperture. In the known HPSEMs with gas scintillation detectors, the ionization density reaches its maximum value close in front of the positive electrode at the end of the secondary electron cascade. With suitable potentials applied to additional electrodes, the result is attained according to the invention that the secondary electron cascade is distributed over a greater volume, so that in all higher amplification factors are attained with uncritical ionization densities at the same time. A substantially higher amplification factor of the secondary electron cascade and of the photons produced by it can be attained with the detectors for HPSEMs according to the invention and the arrangements according to the invention, since the secondary electron cascade is shaped by the second electrode so that the region with the highest ionization density takes up a much larger volume. A much greater amplification factor of the secondary electron cascade and of the photons produced by it can thereby be attained with the same maximum ionization density.
The second disadvantage mentioned hereinabove is avoided by the detector according to the invention, since a secondary electron cascade with high amplification factor is possible between the region close below the final pressure limiting aperture and the electrode at the end of the secondary electron cascade, and therefore a higher amplification factor of the secondary electron cascade does not need to be attained in the region close below the final pressure limiting aperture.
The third problem mentioned hereinabove is solved with the detectors according to the invention by the much greater amplification factor of the secondary electron cascade attainable due to the enlarged volume with large ionization density. Only a slight further amplification by the following operational amplifier is necessary because of this.
In HPSEMs with gas scintillation detectors, the detection efficiency can be improved by a higher amplification factor of the secondary electron cascade. To this extent, the invention is also suitable for HPSEMs with gas scintillation detectors.
In a further advantageous embodiment of the invention, the second electrode or the poorly conducting electrode runs, inclined in the direction toward the sample holder, from the first electrode and forms, at its end toward the sample holder, an aperture for the passage of the field formed by the first electrode or for the field of auxiliary electrodes, which transmits the secondary electrons toward the first electrode.
Further additional auxiliary electrodes are preferably provided for improving the transmission of the secondary electrons through the aperture defined by the end of the second electrode.
Furthermore, the first and second electrodes are made rotationally symmetrical to the optical axis of the scanning electron microscope, i.e., to the beam guiding tube. For this purpose, the first electrode can annularly surround the beam guiding tube, and the second electrode can be formed as a funnel-shaped electrode that surrounds the first electrode and runs conically toward the sample holder. Passage of the field that guides the secondary electrons to the first electrode takes place through the aperture, directed toward the sample, of the second electrode. Negative affects on the primary electrode beam are reduced by the rotational symmetry of the electrode arrangement.
One of the further auxiliary electrodes can be formed as a cylindrical electrode surrounding the beam guiding tube in tubular form.
The first and second electrodes and/or the further auxiliary electrodes can also be constituted as a single electrode with low electrical conductivity, e.g., as an insulator with a poorly electrically conducting coating. Constitution as a solid electrode of a material with low electrical conductivity is also possible, instead of a thin coating on an insulator. Due to the low conductivity of the electrode, a locally different potential is then established along the electrode in the manner of a voltage divider circuit. The end of this poorly conducting electrode facing toward the sample, or the contact at this end, corresponds to the second electrode in the embodiment with separate first and second electrodes. The place, i.e., contact, of the poorly conducting electrode to the highest potential, corresponds to the first electrode in the embodiment with separate first and second electrodes. Therefore, by analogy with the embodiment with separate first and second electrodes, the end of the poorly conducting electrode facing toward the sample is at a lower potential than the place of the poorly conducting electrode with the highest potential.
In a further embodiment of the invention, the poorly conducting electrode is installed in a light guide in the interior of a cavity that is open to one side. The place of contact of this poorly conducting electrode with the highest potential again corresponds in its function to the first electrode. It is situated in the cavity of the light guide at a large distance from the inlet opening on the sample side. The contact of the poorly conducting coating in the neighborhood of the sample-side opening of the cavity corresponds in its function to the second electrode in the embodiment with separate first and second electrodes. The contact of the poorly conducting coating in the neighborhood of the inlet opening of the cavity on the sample side can take place by an electrode which is installed on the outer surface of the light guide and which also serves there as a metallization of the light guide.
The cavity can be constituted in a conical or pyramidal form in order to improve the light conduction in the light guide to a light detector.
However, other shapes of the cavity are possible, for example, a cylindrical bore.
Furthermore, it is possible to constitute the poorly conducting electrode in a region in the neighborhood of the inlet opening with a smaller layer thickness than in a region more remote from the inlet opening. Due to the low conductivity resulting therefrom in the neighborhood of the inlet opening, a higher field strength results in this region than in the region more remote from the inlet opening.
The first electrode, or the contact corresponding to it, extends in a cavity of the light guide, preferably on at least two opposed sides of the cavity, over extended sections in the cavity, in order to distribute the secondary electron cascade over as large a volume as possible.
In an embodiment of the invention in which the volume with high but uncritical ionization density is enlarged in the direction of the direction of propagation of the secondary electron cascade, several electrodes, or a single, only poorly electrically conducting electrode, extend along an elongate cavity or interspace in a light guide or bordering on a light guide.
An xe2x80x9celongate cavity or interspacexe2x80x9d is to be understood as a cavity or interspace whose length is greater than twice the greatest diameter of a circle that can be inscribed in the cross section of the cavity or interspace. In an inlet-side region of the cavity, the electrodes have a potential applied to them such that a high amplification factor results for secondary electrons. Alternatively, additional electrodes can also be provided in front of the inlet opening and can have an applied potential such that a high amplification factor results for secondary electrons. A second region of the cavity or interspace adjoins this inlet-side region, and in it the electrodes have different, weak potentials applied such that in this second region a markedly reduced amplification factor for secondary electrons results, preferably between 0.2 times and 5 times, and ideally a factor of 1.
This second region is an elongate region. No, or no great, amplification of the secondary electron current admittedly takes place in it, but the aim is that a high but uncritical ionization density further remains maintained in it. The secondary electron cascade is constantly attenuated there by impacts in the gas and by the diffusion of the secondary electrons to the walls of the interspace, but is simultaneously amplified again by the adjacent field. A strong gas scintillation occurs in this volume region due to the high ionization density, and can be detected by a photodetector arranged at the end of the light guide. This detector arrangement also makes possible a substantial improvement of the detection sensitivity, by means of an enlarged volume with uncritical ionization density.
A further embodiment consists of a combination with an Everhardt-Thornley detector, in that the end face of the light guide that is present in any case is constructed as a scintillator and is provided with a thin, conductive layer as a contact to which a high voltage can be applied. By means of a further grid electrode mounted in front and largely screening this high voltage from the primary electron beam, the detector arrangement can also be used for electron detection in vacuum operation of the electron microscope.
While in the present usual HPSEMs with collector electrode, a high electric field density is frequently present in the region of the aperture of the final pressure limiting aperture, leading to a marked curvature of the electrical field within the pressure limiting aperture, negatively affecting the primary electrons, and leading to a deterioration of resolution, in the detectors according to the invention, only a weaker electric field strength is required in the region of the aperture of the lower pressure limiting aperture, so that this disadvantageous effect is decreased.
A further embodiment consists in that the axis and/or the midplane of the cavity or interspace is not straight or not planar, respectively, but is curved. First, this has the advantage that many particles that can favor a breakdown, e.g., X-Ray quanta, cannot propagate far in the cavity. Second, this has the advantage that highly energetic electrons cannot well follow the curved course of the field, and therefore, even without collisions in the gas, the highly energetic electrons more easily reach the wall.
An embodiment of such a cavity with a curved axis, preferred because it is easily produced, is (for example) a helical cavity in which the coil can be worked from a cylindrical inner portion and, together with an outer hollow cylinder as the outer portion, forms the helical cavity.